The present invention relates to a structure for separating the high and low pressure turboexpanders of a gas turbine.
In particular, the invention relates to a structure for separating the high and low pressure turboexpanders of a multi-stage axial gas turbine.
The term xe2x80x9cgas turbinexe2x80x9d denotes the whole of a rotary heat engine which converts the enthalpy of a gas to useful work, using gases obtained directly from a combustion process and supplying mechanical power on an output shaft.
The turbine therefore usually comprises one or more compressors or turbocompressors, in which air drawn from the outside is pressurized.
Various injectors supply the fuel, which is mixed with the air to form a fuel-air mixture for ignition.
The axial compressor is driven by a turbine, properly so called, or turboexpander, which supplies mechanical energy to a user by converting the enthalpy of the gases burnt in the combustion chamber.
The turboexpander, the turbocompressor, the combustion chamber (or heater), the output shaft for the mechanical energy, the control system and the starting system form the essential components of a gas turbine machine.
As regards the operation of a gas turbine, it is known that the fluid enters the compressor through a set of inlet ducts.
In these channels, the gas is characterized by low pressure and low temperature, but as it passes through the compressor the gas is compressed and its temperature rises.
It then enters the combustion (or heating) chamber, where its temperature is raised further.
The heat required to raise the gas temperature is supplied by the burning of liquid fuel introduced by injectors into the heating chamber.
Ignition is carried out by sparking plugs when the machine is started.
At the outlet of the combustion chamber, the gas, at high pressure and high temperature, passes through suitable ducts, reaches the turbine, where it gives up some of the energy accumulated in the compressor and in the heating (combustion) chamber, and then flows to the outside through the exhaust ducts.
Since the work transmitted by the gas to the turbine is greater than the work absorbed by the gas in the compressor, a certain quantity of energy remains in the shaft of the machine, and this work, after deduction of the work absorbed by the accessories and by the passive resistance of moving mechanical parts, constitutes the useful work of the machine.
Turbines designed for high power production are generally made with multiple stages to optimize the efficiency of conversion of the energy yielded by the gas into useful work.
Each stage of the turbocompressor and of the turboexpander is designed to operate in certain conditions of pressure, temperature and velocity of the gas.
From the science of thermodynamics it is also known that, in order to obtain the maximum efficiency from any given gas turbine, the gas temperature must be as high as possible, subject to compatibility with the materials that can be used for the components.
The operating conditions can therefore be particularly severe and can cause rapid deterioration of the turbine components in some areas.
In normal operating conditions of a turbine, the damage caused by the breaking of a component can have serious consequences, in addition to the stoppage of the machine for maintenance which is problematic in itself.
In the design of turbines, therefore, cooling systems are provided in critical areas, in order to prevent dangerous rises in temperature.
For example, air can be drawn off from an appropriate stage of the compressor and blown through a system of ducts into the critical area of the stage of the turboexpander for this purpose.
Another problem faced by designers is that of isolating the operating environments of the stages, both in the turboexpanders and in the turbocompressors.
In a turboexpander, for example, an effective isolation system must be provided to separate the high pressure stages from the low pressure stages.
To overcome the aforesaid problems, there is a known way of forming shields between turboexpanders operating and high and low pressure; these shields not only separate the environments, but also have the function of providing a passage for the cooling air to be sent to particularly hot areas of the turbine.
A gas turbine according to the prior art has a high pressure turboexpander AP and a low pressure turboexpander BP, illustrated schematically in FIG. 1 attached to the present description.
The turboexpanders, which operate at different pressures, are separated by a pair of convex plates 2 and 3 made in the form of a circular ring which has a convex surface and is fixed along its outer and inner circumferences.
These convex plates 2 and 3 are fixed and spaced apart in such a way as to form between them a duct 4 for carrying cold air towards the high pressure turboexpander through a set of outlets 5 formed in a central position in the duct 4.
The separating system according to the prior art which has been described has areas for the outlet of the cooling air only in the proximity of the high pressure rotor disc 6 and not in other critical areas of the stage.
Furthermore, the two convex plates 2 and 3 have a drawback in respect of their overall dimensions, which prevent the positioning of the high and low pressure rotors close to each other because they require the use of a transition element 7 between the two turboexpanders, resulting in a pressure drop and consequently a decrease in the efficiency of the turbine.
The object of the present invention is to provide a structure for separating the high and low pressure turboexpanders of a gas turbine, which is free of the drawbacks mentioned above.
Another object of the present invention is to propose a separating structure which enables cooling air to be directed towards hot areas within the gas turbine, such as the high and low pressure wheel housings and the high pressure rotor disc rod.
Another problem associated with turbines consists of the losses due to leakage between the high and low pressure environments.
These losses include a reduction of efficiency which, although less significant than other characteristic losses, such as the kinetic energy of the exhaust gas, friction in the ducts, the windage, etc., causes a departure from the optimal operating conditions of the turbine.
A further object of the present invention is to provide a support for the sealing ring which isolates the high and low pressure environments.
These and other objects, according to the invention, are achieved by the structure for separating the high and low pressure turboexpanders of a gas turbine as disclosed in claim 1.
Further characteristics of the separating structure according to the invention are specified in the subsequent claims.
The structure for separating the high and low pressure turboexpanders according to the present invention comprises a diaphragm for receiving cooling air drawn from a stage of a turbocompressor, a pair of shaped plates bolted at one end to the diaphragm, and a sealing ring bolted to the other ends of the shaped plates and supported by them.
The structure provides a separation between the high and low pressure stages and, because of the gap formed by the space between the shaped plates, allows the cooling air to be conveyed through the diaphragm towards hot areas of the turboexpanders.